Читать книгу Honey Bee Medicine for the Veterinary Practitioner - Группа авторов - Страница 25

Seeley and Morse (1976) reported that the average wall thickness of natural nest cavities is approximately 20 cm (~8 in.). The wall thickness of a standard Langstroth hive is just 1.9 cm (0.75 in.), hence some 10 times thinner than the nest cavity wall of a bee tree. The reduced wall thickness in Langstroth hives creates a large reduction in nest insulation, possibly resulting in adverse effects on colony energetics. Large temperature fluctuations inside a hive exacerbate colony stress by increasing the demands on colony nutrition and hydration (more nectar and water foraging trips), by impairing a colony's ability to maintain thermal homeostasis (more fanning and “bearding” when it is hot, and more metabolic heat production when it is cold), and by hastening entry into a winter cluster – all of which increase the physiological demands on the colony (Mitchell 2016).

Coombs et al. (2010) found that natural tree cavities buffered environmental temperatures such that tree cavities were cooler than ambient during the day and warmer than ambient during the night. During the day, the tree diameter at breast height was the most important variable determining cavity temperature. At night, diameter and tree health were important with large living trees offering the most stable thermal environment. We compared the ambient temperatures inside two tall, man‐made cavities; one was inside a rectangular wooden box (built of 1.9 cm thick pine boards, as used for Langstroth hives) and the other inside a living sugar maple tree (Acer saccharum) (Figure 1.5). These two cavities were built with the same dimensions (24 cm × 24 cm × 87 cm), which mimicked those of a typical tree cavity of a wild colony [see Tree Beekeeping by Powell (2015)]. Temperature recordings over a year revealed striking differences in interior temperature dynamics between the two cavities. In the poorly insulated box, the temperature closely followed the ambient temperature; the thin walls provided little or no temperature buffering. In the tree, though, the temperatures varied much less; they did not reach the extreme highs and lows found inside the uninsulated box (Seeley and Radcliffe unpublished data; see Figure 1.6a,b).

Figure 1.4 An illustration comparing the structure and organization of a honey bee nest as found in a bee tree (left) and a standard Langstroth hive made up of two deep hive bodies (right). The colors correspond to brood and hive products. A typical bee tree cavity has a volume averaging 40 l, whereas two deep hive bodies have a volume of 80 l. These differences in cavity volume are directly correlated with the size of a colony's brood nest and varroa reproductive success.

Mitchell (2016) found that heat is transferred four to seven times faster across the thin walls of a traditional hive relative to the walls of a natural (bee tree) enclosure. To maintain a colony's cluster core temperature of 35 °C (the set point of the brood nest), any energy lost through transfer from the hive walls must be replaced through the bees' metabolic activity (bees isometrically contract their flight muscles to generate heat). Mitchell predicted that colonies living in hives (or trees) providing well‐insulated cavities will not need to assemble into tight clusters until the ambient temperature is below 0°C. Mitchell concluded that the high thermal insulation of nests in bee trees results in increased relative humidity inside the cavity, decreased reproduction by Varroa mites, and enhanced survival of honey bee colonies.

Figure 1.5 A research station beside the Shindagin Hollow State Forest in upstate New York. It was designed to test the environmental fluctuations – temperature (°C) and relative humidity (%) – inside two cavities of identical dimensions but with walls of different thicknesses, c. 2 cm vs. 20–30 cm. One (a) is a wooden box with walls like those of a Langstroth hive and the other (b) is a live sugar maple tree (Acer saccharum) in which a typical size bee cavity was cut using a chainsaw and adze. Source: Photo by Robin Radcliffe.